ALBERT

Description: European windstorms are a high-impact weather phenomenon, regularly inflicting substantial damages, both human and economic. This study examines a set of objectively selected intense European windstorms from the 1979–2015 period using re-analysis and forecast products from the European Centre for Medium-Range Weather Forecasts (ECMWF). The storms are first categorised with respect to their diabatic relative to the baroclinic contribution to deepening using the pressure tendency equation and additionally with respect to their track relative to the jet stream as the large-scale element controlling stormdeepening and propagation. As expected, baroclinic processes dominate the majority of storms, such that deepening is closely related to warm advection ahead of the cyclone centre. Contributions from diabatic processes vary strongly and exceed those from horizontal temperature advection in 10 out of the 58 cases with values of up to 60%. Remarkably in several cases, planetary waves in the stratosphere appear to facilitate cyclogenesis, but can also act to oppose deepening in a few cases. The diabatic contribution is significantly correlated to the time a given storm spends on the equatorward side of the jet, where there is greater potential for diabatic processes in the warm, moist air. In terms of forecast quality, and consistently with previous studies, the storms’ core pressure is generally underestimated and storms tend to be too slow and shifted south in the forecast, particularly for longer lead times. These biases, however, reduce markedly with the improvement of the operational system over time. There is no systematic dependency of forecast behaviour on diabatic contribution or track relative to the jet. In the future, some of these analyses should be repeated with homogeneous reforecast data to better substantiate these findings.

Description: Very sparse data has previously limited observational studies of meteorological processes in the Sahara. We present an observed case of convectively-driven water vapour transport crossing the Sahara over 2.5 days in June 2012, from the Sahel in the south to the Atlas in the north. A daily cycle is observed, with deep convection in the evening generating moist cold pools that fed the next day's convection; the convection then generated new cold pools, providing a vertical recycling of moisture. Trajectories driven by analyses were able to capture the direction of the transport but not its full extent, particularly at night when cold pools are most active, and analyses missed much of the water content of cold pools. The results highlight the importance of cold pools for moisture transport, dust and clouds, and demonstrate the need to include these processes in models in order to improve the representation of Saharan atmosphere. [150]